1. Field of the Invention
This invention relates to an electronic shutter for use in a camera and in particular to a shutter in which a movable electromagnetic armature cooperates with an opening and a closing shutter blade to effect an electronically controlled, variable exposure duration and a mechanically controlled minimum exposure duration.
2. Discussion of the Prior Art
Camera shutter mechanisms are provided in the prior art which include two blades that are slidably mounted and moved in sequence to effect exposure. The opening blade is released to begin exposure and the closing blade is electromagnetically delayed for an interval related to scene brightness, whereafter the closing blade is released to terminate exposure. The electromagnetic delay is generally provided by an electromagnetic latch which includes an electromagnet and an armature movable in response thereto, which latches the closing blade as long as the electromagnet is energized. If the electromagnet should fail to operate because, for example, of the failure of the power supply, both blades will move simultaneously, and will not uncover the exposure aperture for an interval sufficient to provide an acceptable photographic exposure. Some shutter mechanisms correct this deficiency by employing, in addition to an electromagnet delay device, an inertial-retard device that operates after release of the closing blade to retard its movement for a predetermined minimum exposure interval. Other shutter mechanism includes a movable stop that engages the closing blade and which is released by the opening blade after initial movement thereof.
While these corrective measures insure at least a minimum exposure interval, these measures complicate the camera construction and require at least one additional operating element that is also subject to mechanical failure. The present invention minimizes additional elements by utilizing the opening shutter blade and the armature as a detent for holding the closing blade for a preset time in relation to the movement of the opening blade. Thereby the release of the electromagnet latch is prevented for a predetermined time and thus an exposure of fixed duration is produced.